In nuclear power plants, check valves are utilized in various fluid circuit configurations, such as to control the flow of coolants to the fuel core and in emergency safety systems.
One circuit configuration provides a check valve between the reactor vessel and a high pressure core injection system. This system replaces small quantities of water lost from the reactor if the reactor pressure is only slowly reduced.
Another core cooling system is the low pressure core injection system in which large volumes of water at low pressure are injected into the vessel to restore water level to at least mid-core level.
These safety injection systems supply water from a storage tank through appropriate pumps to any leg of the primary loop. The use of check valves in these configurations is somewhat different from normal check valve usage, and those accustomed to thinking of check valves in their normal context, on the discharge side of pumps, will have to mentally adjust to the particular use of check valves on these safety injection systems. Conventionally, on a pump discharge, the check valve is normally open to flow. Whenever the pump stops, the check valve closes automatically and prevents back flow of water through the pump. On these safety systems, however, the valves are normally closed and held closed by high system pressure in the reactor vessel acting on the outlet or downstream side of the valve disc. When an emergency occurs, it results in lowering the system pressure to a point lower than that of the safety injection system. When this occurs, the check valve opens, permitting flow of the cooling water into the reactor vessel.
The check valves are installed in tandem pairs in each circuit to form a double barrier which provides a redundancy in case of failure of one of the valve discs. It is feared, however, that these installations do not ensure that both check valves reseat or that the sealing integrity of both valves would be effective barriers. It is possible that one valve could be stuck open or have a leaky seal and thus leave only one barrier effective during plant operation.
Similarly, because the check valves are held shut for normally long periods of time with considerable pressure differential pressing the disc against the seat, corrosion products may form between seat and disc and prevent the disc from opening when called upon.
Another problem encountered in these safety injection systems is water hammer caused by reverse flow through the check valves where the valve disc "slams" shut causing a high pressure surge. The primary loops utilize canned multi-stage pumps which are high speed compact units with low pump and motor inertia. When the pump power cuts off, the water in that leg slows down very rapidly and may establish reverse flow in fractions of a second. To prevent or minimize this reverse flow, the check valve must have extremely fast response and this is especially important in these multi-pump systems. A typical reactor fluid circuit would have several primary loops, each with their own pump, check valves and heat exchangers, with each loop leading to the common reactor vessel. If power is cut off to one pump, the remaining pumps will maintain a high differential pressure in the common reactor. This would cause the column of water in the de-energized loop to decelerate very rapidly. With a check valve having slow response, the flow in this loop could reverse and drive the idle pump in reverse.
From the foregoing, certain requirements have been imposed on check valves for nuclear reactor service. They must pass an "in service" air leakage test with 45 or 50 psig (1" diameter maximum conduit) which assures valve tightness and the ability to isolate and prevent radioactive fluid from seeping into other pipe systems during plant shutdown and maintenance. They must have a remotely controlled exercisable actuator to open the valve 20.degree. to 30.degree., with a balanced pressure on both sides of the valve disc, and provide assurance that the valve disc is not sticking and is free to operate. Also they must have a "non-slam" characteristic and maintain low pressure surges during closure. Finally, experiments with existing nuclear plants, and confirmed by additional experiments and research, show that a properly designed tilt disc check valve, in comparison with a swing check valve, produces one-half to one-third less pressure surge.